She was questioned by the Senate Foreign Relations committee about September's assault on the US consulate in Benghazi in which ambassador Christopher Stevens and three other Americans were killed.

Accepting responsibility for missteps by her department but rejecting talk of a cover-up in its wake, a confident Mrs Clinton, pictured, nevertheless became irate when Ron Johnson, a Tea Party-backed senator, pressed her on the length of time the administration took to acknowledge that the attack in Libya was the work of terrorists and not an outgrowth of a street protest over an American anti-Islam video.

The exchange with Mr Johnson cut to the heart to the hearings that had been delayed for several weeks to allow Mrs Clinton to recover from a blood clot in the brain.

Did the administration try to obscure the terrorist nature of the attack to minimise possible damage to Barack Obama as he sought re-election?

"The fact is we had four dead Americans," she said, gesticulating angrily.

"Was it because of a protest? Or was it because of guys out for a walk one night who decided they'd go kill some Americans? What difference, at this point, does it make?

"People were trying in real time to get to the best information."

Emotion

In her opening statement, Mrs Clinton showed unusual public emotion as she recalled the days after the attack.

"I stood next to President Obama as the Marines carried those flag-draped caskets off the plane," she recalled, her voice cracking.

"I put my arms around the mothers and fathers, sisters and brothers, sons and daughters."

She told the senators that the State Department is enacting all of the 29 recommendations contained in an independent report into the attack.

Mrs Clinton used her time to also underscore the growing concern in Washington about events in north Africa generally. In both Mali and Algeria, she said, guns coming out of Libya were implicated.

"This Pandora's box of weapons coming out of these countries of the Middle East and north Africa is the source of one of our biggest threats."

Nor did she leave without first drawing attention to Syria, saying that any attempt by the regime there to use biological or chemicals weapons remained a "red line" for America.

While Mrs Clinton said she accepted ultimate responsibility for the fact that the Benghazi facility had been insufficiently protected, she rejected claims that she should have known about security requests that went ignored.